PSNB 1.4

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1.4 Sea-Floor Spreading


Vocabulary

Mid-Ocean Ridge: The longest chain of mountains in the world.
Sonar: A device that records the echo, by bouncing sound waves of under water objects.
Sea-Floor Spreading: The process that keeps adding new material to the ocean floor.
Deep-Ocean trenches: Where the ocean's crust bends backward.
Subduction: Where the ocean floor sinks beneath an underwater trench.

Outline

  • Mapping the Mid-ocean Ridge

    • Longest chain of mountains in the world
    • Lies underwater, some parts poke above surface
    • Scientists map it with sonar
    • Iceland is a part of the ridge
    • Ridge's existence made scientists curious, they investigated further

  • Evidence for sea-floor spreading

    • Harry Hess studied mid-ocean ridge
    • He thought the ocean floor moves like a conveyer belt, carrying continents along
    • Also thought that molten material comes out of the ridge, spreading out and pushing older rock to both sides of the ridge.
      • believed this molten material cools and forms a strip of rock in center of ridge
  • Evidence From Molten Material
    • Scientists went on a submersile named Alvin to see evidence
    • They saw pillow/toothpaste type rocks, they can only be formed if molten material has hardened quickly
  • Evidence From Magnetic Stripes
    • Earth's magnetic poles reversed themselves 780,000 years ago
    • New rock from molten material is always pointing north, so when poles reversed, they were pointing different ways.
      • Scientists saw this and started believing sea-floor spreading
  • Evidence From Drilling Samples
    • Scientists drilled holes in ocean floor using a ship, they found the youngest rocks were in the center, the oldest rocks were away from the ridge

  • Subduction at deep ocean trenches

    • Old oceanic crust sinks into deep ocean trenches (underwater canyons)
  • Subduction In the pacific ocean
    • The pacific ocean is shrinking because the ridge can not produce as much crust as the big trenches are swallowing
  • Subduction in the Atlantic Ocean
    • The Atlantic Ocean is growing, because the trenches are too small, so they can not swallow as much as the ridge is creates




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